Learn Finnish: Tätä tuotetta myydään yksittäin. - This product is sold individually.

It is built from the stem of yksi (one), which is yks-, plus the distributive suffix -ittäin. This suffix expresses how something is grouped or distributed (meaning "in groups of X" or "by the X"). Therefore, yksittäin literally translates to "in ones", "one by one", or "individually".

Why does the suffix use the letter 'ä' (-ittäin) instead of 'a' (-ittain)?

This is due to vowel harmony. The root word yksi contains the front vowel y (along with i, which is neutral). Because of the y, the suffix must also use the front vowel ä rather than the back vowel a.

Why is the object Tätä tuotetta in the partitive case?

The verb myydä (to sell) takes a partitive object when the action is ongoing, continuous, or describing a general practice. Because this sentence describes a general rule about how the product "is sold" rather than a completed, single event, the object must be in the partitive.

What verb form is myydään?

It is the present passive form of myydä (to sell). The Finnish passive is used when an action is performed by a general, unspecified "someone" or "people". It maps perfectly to the English passive in this sentence: "is sold".